1. Field of the Invention
The present invention relates to a light receiving device using a new sensitivity control method and a spatial information detecting apparatus using intensity-modulated light according to the technical concept of the same light receiving device.
2. Disclosure of the Prior Art
In the past, to control the sensitivity of a light receiving device, it has been known to place an optical member adjacent to the light receiving device. This optical member has the capability of adjusting a light amount supplied to the light receiving device according to an external signal. In addition, it has been already introduced to adjust a ratio of amounts of electric charges output as signal charges from the light receiving device to the amounts of electric charges generated in the light receiving device according to an intensity of received light in order to control the sensitivity of the light receiving device.
The technique of controlling the sensitivity of the light receiving device can be used for an apparatus for detecting spatial information. For example, an intensity-modulated light is irradiated from a light source into the space, and a reflection light reflected from an object in the space is received by the light receiving device. According to a relationship between the intensity-modulated light and the received light, the spatial information can be determined. In the present specification, the spatial information includes a distance between the apparatus and the object in the space, a change in an amount of received light caused by reflections on the object in the space, and so on. For example, the distance between the apparatus and the object can be determined from a phase difference between the intensity-modulated light and the received light. In general, this technique is called the time-of-flight method.
U.S. Pat. No. 5,856,667 discloses an apparatus using this time-of-flight method. In this apparatus, the light emitted from the light source is intensity-modulated by a sine wave having a required modulation frequency (i.e., emission frequency), and a light sensitive part detects the intensity of received light plural times within a time period shorter than a modulation period that is the reciprocal of the modulation frequency. For example, the intensity of received light is detected by the light sensitive part at a certain phase of the modulated light. For example, the intensity of received light can be detected 4 times within one modulation period, and the phase difference is determined from the detected four intensities of received light.
When detecting the intensity of received light at the specific phase within the modulation period, it is needed to control the sensitivity of the light receiving device. In the apparatus disclosed in U.S. Pat. No. 5,856,667, a switch area is formed between the light sensitive part for producing a number of electric charges when receiving the light and a memory area for storing the generated electric charges, and required amounts of the electric charges are transferred from the light sensitive part to the memory area through the switch area. However, since the switch area and the memory area are formed independently from the light sensitive part, there is a problem that the apparatus becomes a complex structure as a whole. In addition, since light shielding is needed for the switch area and the memory area, there is another problem that the light shielding area relatively increases to reduce an aperture ratio, so that the sensitivity lowers.